1. Field of the Invention
The present invention relates to industrial fabrics, particular paper machine clothing e.g. as forming fabrics, dryer fabrics or base cloths of press felts.
2. Discussion of Background Information
Paper is conventionally manufactured by conveying a paper furnish, usually consisting of an initial slurry of cellulosic fibres, on a forming fabric or between two forming fabrics in a forming section, the nascent sheet then being passed through a pressing section and ultimately through a drying section of a papermaking machine. In the case of standard tissue paper machines, the paper web is transferred from the press fabric to a Yankee dryer cylinder and then creped.
Papermachine clothing is essentially employed to carry the paper web through these various stages of the papermaking machine. In the forming section the fibrous furnish is wet-laid onto a moving forming wire and water is encouraged to drain from it by means of suction boxes and foils. The paper web is then transferred to a press fabric that conveys it through the pressing section, where it usually passes through a series of pressure nips formed by rotating cylindrical press rolls. Water is squeezed from the paper web and into the press fabric as the web and fabric pass through the nip together. In the final stage, the paper web is transferred either to a Yankee dryer, in the case of tissue paper manufacture, or to a set of dryer cylinders upon which, aided by the clamping action of the dryer fabric, the majority of the remaining water is evaporated.
Industrial fabrics like Paper machine clothing are mainly manufactured by weaving. The yarns used for weaving can be for example of single or twisted monofilament, multifilament or spun bound type. Materials used are based on polyester, polyamide or polyphenylene sulphide (PPS).
The weaving process is characterized in that the finished fabric comprises interwoven warp and weft yarns, whereby the warp and weft yarns cross over each other at cross-over points resulting in the fact that a woven fabric never can have totally flat surfaces. Therefore fabrics often are characterized by surface features that are predominantly made up of warp or weft dominated arrays.
For some applications it is desirable to have fabrics with flat surfaces. E.g. in the dryer section one function of the dryer fabric is to give sufficient heat transfer from the heated surface e.g. of a drying cylinder to the sheet of paper. This is typically achieved by sandwiching the paper sheet between the dryer fabric and the drying cylinder. The effectiveness of the heat transfer is determined by factors such as pressure applied to press the sheet against the heated cylinder and the contact density (contact area and contact points), that means the contacting surface between the dryer fabric and the sheet.
A drawback of woven fabrics is that they are showing the property of “crimp” caused by the over and under arrangement of the warp and weft yarns. After the weaving process mainly the warp yarns are crimped. During the heat stabilizing process, where heat and tension simultaneously is applied to the fabric, some of the crimp is lost from the warp yarns but imparted into the weft yarns, this is called “crimp interchange”.
Fabrics have to exhibit uniform properties for example characterized by their vapour and/or water permeability, caliper, surface topographie, tension, dimensional stability etc. through their entire length and width. These properties have to maintain stable over their entire life time. Sometimes the performance of woven fabrics in maintaining properties over their life is not satisfactory.
As a result from the weaving process, the woven fabric has a woven structure with channels for water and vapour passage resulting in a certain water and vapour permeability of the fabric. In the forming and pressing section of a paper making machine mainly the water permeability of the fabric is important to control the liquid dewatering and to avoid rewetting of the sheet. In the dryer section mainly the vapour permeability of the fabric is important to control the passage of moisture vapour from the sheet through the fabric.
Further woven fabrics are not easy to clean because of their complex 3-dimensional open structure. This issue becomes more and more important due to the fact that within the paper making process there is a constant drive towards more and more recycled material to be used including more contaminants. This leads to increased contaminations of the fabric.
To overcome some of the above mentioned drawbacks non woven fabrics have been proposed.
U.S. Pat. No. 3,323,226 describes a synthetic dryer fabric made by mechanical perforating polymeric sheet material.
U.S. Pat. No. 4,541,895 describes a paper makers fabric made up of a plurality of impervious non-woven sheets joined together in a laminated arrangement to define the fabric or belt. Defined throughout the fabric are drainage apertures which are created by drilling techniques.
GB 2 235 705 describes a method for manufacturing a non-woven fabric where an array of sheath core yarns of which the core has a higher melting point than the sheath, is fed in spaced parallel disposition to peripheral grooves of a pinned roller arranged in nip forming relationship with a press roll. Thereby the material of the sheath is melted as the yarns move into and through the roller nip and excess melted sheath material is forced into lateral grooves in the roller to form structural members between adjacent yarns.
All the above mentioned non-woven structures are showing unsatisfactory dimensional and thermal stability.